Before, and even after a casing is installed in a wellbore, the well may be treated or stimulated. Stimulation involves pumping stimulation fluids such as fracturing fluids, acid, cleaning chemicals, and/or proppant laden fluids into the formation to improve wellbore production. The stimulation fluids are pumped through the casing and then into the wellbore. If the casing is installed and more than one zone of interest of the formation is treated, tools must be run into the casing to isolate fluid flow at each zone.
Instead of stimulating the formation after installing casing, the well operator may choose to stimulate an uncased portion of a wellbore. To do so, the operator may run a liner extending from the surface into the uncased section of the wellbore with inflatable element packers to isolate the portions of the wellbore. Multiple packers allow the operator to isolate segments of the uncased portion of the wellbore so that each segment may be individually treated to concentrate and control fluid treatment along the wellbore.
The tubing string, which conveys the treatment fluid, can include ports or openings for the fluid to pass into the wellbore. Where more concentrated fluid treatment is desired in one position along the wellbore, a small number of larger ports may be used. Where it is desired to distribute treatment fluids over a greater area, a perforated tubing string may be used having a plurality of spaced apart perforations through its wall. The perforations can be distributed along the length of the tube or only at selected segments. The open area of each perforation can be pre-selected to control the volume of fluid passing from the tube during use.
Another method of treating a formation with or without an uncased wellbore involves running a non-casing fluid treatment tubing string with packers into the wellbore. The string includes at least one section of ports that are openable when desired to permit fluid flow into the wellbore. A sleeve or sleeves are located inside the tubing at each section of ports in the tubing and include ports that correspond with the ports in the tubing. The sleeves are initially axially offset from the tubing ports so that the tubing ports are closed to fluid flow. The sleeves include annular seats of differing diameters. To open a given set of ports, at least one packer is set to isolate the annulus between the tubing string and the formation or casing around the section of ports. A ball is then pumped down and landed on the annular seat of the given sleeve. If more than one sleeve is used, the diameters of the annular seats are staged with decreasing diameters. Thus, a ball with a diameter for landing on the given sleeve will pass through the annular seats of any previous sleeves as is passes through the tubing. With the ball landed on the annular seat of the desired sleeve, fluid pressure is applied to form a seal preventing fluid flow past the sleeve. The fluid pressure also moves the sleeve axially, thus matching up the ports in the sleeve with the ports in the tubing and allowing fluid flow from the tubing to pass through the sleeve ports, through the tubing ports, and into the wellbore.